1 kg strong flour (alternately 750g plain flour combined with 250g rice flour)
2-400g chopped macadamia nuts (aiming for an even consistency, with the occasional chunky bit)
Ensure the butter is chilled. Combine the flour and sugar evenly in a large mixing bowl. Chop the butter into cubes, and rub it into the flour/sugar mix as lightly as practical. Once the mix has the texture of breadcrumbs and comes away from the side of the bowl in clumps, work the chopped nuts through.
Grate the zest off of all the lemons and limes, and mix this through as evenly as possible. Get as much of the juice out of the fruit as practical, and water it down by about 50/50 juice/water. Make a well in the mixture, and add small quantities of juice/water, mixing and kneading as you go, until the mixture binds coherently. Given the amount of shortening, the amount of liquid actually required to achieve this should be fairly small, not more than 250ml or so.
Knead the resulting dough till smooth, then cover (or wrap in cling film) and refrigerate for at least 30 minutes.
Preheat an oven to 200C. Take a portion of the dough, roll it into a thick sausage 4-5cm in diameter. With a sharp knife, cut roughly 1cm disks from this roll. Place the disks on a baking tray (I used muffin trays this year, the disks fit pretty neatly). Bake for either 20 mins on 180C or 15 minutes on 200C, depending on how crumbly you like your shortbread.
(Many thanks to Damian for this.)